1. Field of the Invention
The present invention relates to a positioning control apparatus having a reference position learning function, and more particularly, it relates to a positioning control apparatus having a reference position learning function so as to improve control accuracy without being influenced by aged deterioration, mounting conditions, etc.
2. Description of the Related Art
Conventionally, there has been known a device capable of performing reference position learning in a positioning control apparatus having a position sensor such as a potentiometer, as described in Japanese Patent Application Laid-Open No. 5-332191 entitled “Throttle Reference Opening Detection Device”, for example. This device learns a reference position in order to prevent the deterioration of control accuracy due to mounting play or clearances in driving parts and a position sensor, the extension of a wire in case of an actuator using the wire, etc.
In the reference position learning, a fixed stopper is installed at a position which becomes a reference, and a positioning control object, i.e., an object whose position is to be controlled, is continuously driven to move in a direction toward the fixed stopper until it is placed into abutment with the stopper. The reading of a position sensor when the positioning object becomes unable to move any more is learned as a reference position.
Even with a change in the mounting state of the position sensor, it is always possible to detect the accurate position of the positioning object from the reference position by converting, through calculations, the output value or reading of the position sensor into a relative position with respect to the reference position. Accordingly, by performing accurate positioning control using the relative position, it becomes possible to carry out the positioning control in an accurate manner.
The positioning control apparatus having the conventional reference position learning function as shown in the above-mentioned publication determines the reference position by bringing the positioning object into abutment against the stopper in the reference position learning. The positioning object might not be able to be driven to move in the direction of the stopper by some reasons. For instance, in cases where the positioning object is unable to be temporarily driven, upon starting of its operation, in a direction toward the stopper due to causes such as biting of foreign matter, carbon clinging, etc., the initial position of the positioning object is learned as a reference position even if the initial position is far away from an original or inherent reference position. As a result, the following positioning control might be influenced harmfully.
Moreover, it is general to set a relatively large control gain in the positioning control so that the positioning object can reach a target position as quickly as possible. That is, a driving force for driving the positioning object is often set to a large value. In such a condition in which the driving force is large, if the target position for positioning control is set at a location at which the positioning object is unable to reach the stopper even when it overruns the target position (e.g., the stopper position may be set at a location within a movable range and outside an area in which the positioning target is usually set.), there will be no necessity for considering damage to a driving mechanism such as a motor, gearing and the like for driving the positioning object, due to an impact which would otherwise be generated when the positioning object is placed into abutment against the stopper. However, the abutment of the positioning object against the stopper is indispensable for the reference position learning, and hence there will be damage to the driving mechanism due to an impact caused by the abutment of the positioning object against the stopper.